This invention relates generally to the packaging of semiconductors. More particularly, this invention relates to a technique for integrating a semiconductor and a heat spreader for low thermal resistance and improved lateral heat transfer.
It is becoming increasingly difficult to efficiently dissipate heat from semiconductors as the size and transistor density of semiconductors grow. It is known to use heat spreaders to remove heat from semiconductors. As used herein, the term heat spreader refers to a metallic element, such as a metallic slab, a contoured heat sink, and the like, used for heat transfer. The effectiveness of a heat spreader is contingent upon forming an efficient thermal joint with its associated semiconductor.
Epoxy is sometimes used between a semiconductor and a heat spreader. Unfortunately, epoxy results in relatively high thermal resistance. In addition, the epoxy provides practically no lateral heat dissipation. Lateral heat dissipation is becoming increasingly important as modern semiconductors experience large variances in heat flux across a semiconductor die. Lateral heat dissipation is required to transport heat from high heat flux density locations to low heat flux density locations.
To overcome the problems associated with epoxy, solder has been used to attach a heat spreader to a semiconductor. While solder provides relatively low thermal resistance compared to epoxy, there are a number of difficulties associated with its use. First, solder attachment requires high processing temperatures in the range of 150xc2x0 C. to 350xc2x0 C., depending upon the alloy. These relatively high temperatures can result in differential expansion between the heat spreader and the semiconductor. Therefore, when the high temperature attachment process is completed and the package is brought down to room temperature, the mismatch in coefficients of thermal expansion can produce a void between the heat spreader and the semiconductor. The high temperature attachment process associated with solder also requires special equipment to contain the molten solder. In addition, special equipment must be used to prevent the formation of native oxides at the inter-metallic joint.
Attempts have been made to form a heat spreader directly on a semiconductor. Unfortunately, attempts to directly deposit a metal on a semiconductor have resulted in poor adhesion. Thus, during temperature cycling, mismatches in the coefficients of thermal expansion between the heat spreader and the semiconductor have resulted in fracturing between the heat spreader and semiconductor. This problem is exacerbated as the size of a semiconductor increases.
In view of the foregoing, it would be highly desirable to provide an improved technique for integrating a semiconductor with a heat spreader for low thermal resistance and improved lateral heat transfer.
The invention includes a method of integrating a heat spreader into a semiconductor. The method includes depositing an adhesion metal layer on the back of a semiconductor at low temperature. A heat transfer metal layer is subsequently deposited on the adhesion metal layer at low temperature to form a heat spreader.
The invention also includes a semiconductor with an integrated heat spreader. The apparatus has a semiconductor with an active side and a passive side. An adhesion metal layer is attached to the passive side of the semiconductor. A heat transfer metal layer is positioned on the adhesion metal layer to form a heat spreader.
The technique of the invention provides atomic level bonding between the semiconductor and the adhesion metal. In turn, the heat transfer metal layer forms a tight bond with the adhesion metal. The heat spreader of the invention is deposited in a fully annealed condition, resulting in little intrinsic stress at the joint with the semiconductor. Thus, the heat spreader of the invention establishes a tight joint with its substrate to resist de-lamination and to facilitate heat transfer away from the semiconductor. The heat spreader of the invention facilitates extended lateral heat transfer with low thermal resistance. Therefore, excessive heat from high heat flux regions of a semiconductor is distributed to low heat flux regions of the semiconductor.